It has been known for some time that certain types of radiation, such as ionizing radiation and actinic light, can be used to effect free radical-induced polymerization of certain materials. However, because the energy utilization in such polymerization methods is quite low, and because the response of materials which one would expect to be useful in such methods varies and often is unexpectedly so low as to make the method impractical, little interest has been shown in polymerization by means of such radiation until relatively recently.
The types of materials which are theoretically polymerizable by means of radiation are limited to those which react by a free radical-induced mechanism, and thus this process is not applicable to materials which form by condensation reactions. The production of many highly useful polymeric materials, such as polyurethanes, polyamides, aminoplast resins and the like, therefore ordinarily cannot be carried out by means of these processes. For the most part, polymerization using radiation involves liquid monomeric species having polymerizable ethylenic groups and which have a suitable viscosity in the absence of any solvent.